1. Field of the Invention
The present invention relates to a step-up inverter transformer used in an output stage of an inverter for turning on a light source to illuminate a liquid crystal display.
2. Description of the Related Art
Recently, as display means for personal computers or the like, a liquid crystal display (hereinafter referred to as LCD) has been increasingly taking the place of a cathode ray tube (hereinafter referred to as CRT). The LCD, unlike the CRT, does not have a light emitting function, and therefore needs a backlight- or frontlight-type light source.
In order to illuminate an LCD screen brightly, two or more cold cathode fluorescent lamps (hereinafter referred to as CFL), which are simultaneously arc-discharged and lighted, may be used as the aforementioned light source.
In general, to discharge and light such CFLs, an inverter circuit is used in which a DC voltage of about 12 V is supplied through a Royer-type oscillator to the primary side of a transformer (inverter transformer) as an AC voltage, and in which a high frequency voltage of about 1600 V with 60 kHz is generated at the secondary side at the start of discharging.
After discharging of the CFLs, the inverter circuit controls the secondary-side voltage of the inverter transformer to be reduced to about 600 V required fir keeping the CFLs discharging. For this voltage control, pulse width modulation (hereinafter referred to as PWM) control is usually employed.
In such an inverter circuit, an open-magnetic-circuit inverter transformer using a bar-shaped core as a magnetic core, and a closed-magnetic-circuit inverter-transformer have been conventionally used.
FIG. 22 shows an equivalent circuit of an open-magnetic-circuit inverter transformer. In the figure, reference numerals 1, L1, and Ls denote an ideal step-up transformer (inverter transformer) with a winding ratio of 1:n and without loss, a leakage inductance, and an inductance of a secondary winding, respectively. When one CFL 2 is connected to the ideal step-up transformer (open-magnetic-circuit inverter transformer) 1, the leakage inductance L1, works as a ballast inductance and discharges normally. However, when two CFLs 2 are connected in parallel to inverter transformer output terminals T, and when one CFL 2 of the two starts discharging before the other CFL 2, the voltage at the output terminals T is reduced due to the leakage inductance L1, failing to allow the other CFL 2 to discharge.
FIG. 23 shows an example of the open-magnetic-circuit inverter transformer 1 which uses a bar-shaped core 3 as a magnetic core. The bar-shaped core 3 is inserted into a hollow 5 of a tubular bobbin 4 as shown by a dashed line. The bobbin 4 has a primary winding 6 and a secondary winding 7 wound thereon, and has a terminal block 9 with terminal pins 8 of the primary winding 6 and a terminal block 11 with terminal pins 10 of the secondary winding 7. Since the voltage induced at the secondary side is high, the secondary winding 7 is sectioned by partitions 12 provided on the bobbin 4 to prevent creeping discharge.
The open-magnetic-circuit inverter transformer 1 with the bar-shaped core 3 as a core is of a simpler structure than a closed magnetic circuit inverter transformer 1A, in which, as shown in FIG. 24, a rectangular frame-shaped core 13 and a bar-shaped core 3 are coupled to form a magnetic core, and primary and secondary windings 6 and 7 are provided on a bobbin 14 in which the bar-shaped core 3 is inserted. In the inverter transformer 1, however, since the leakage inductance is large, when a plurality of CFLs are connected thereto, it may happen that only one CFL is turned on with the rest failing to be turned on.
The closed-magnetic-circuit inverter transformer 1A shown in FIG. 24 is configured such that the bar-shaped core 3 is inserted in a hollow of the bobbin 14, the primary and secondary windings 6 and 7 are wound on the bobbin 14, and that the bobbin 14 is fitted into grooves 15 of the rectangular frame-shaped core 13.
The inverter transformer 1A shown in FIG. 24 may be configured as an open-magnetic-circuit type by providing a gap between the rectangular frame-shaped core 13 and the bar-shaped core 3, whereby the leakage inductance can be controlled. However, when a plurality of CFLs are connected in parallel, it may happen that all the CFLs are not turned on simultaneously. Accordingly, in an open-magnetic-circuit inverter transformer, one inverter transformer is necessary for each of the plurality of CFLs in order to turn on all the CFLs simultaneously.
When a plurality of CFLs are used in order to illuminate a screen of LCD brightly, a plurality of inverter transformers are required, resulting in an increased size as a whole and also an increased cost.
The open-magnetic-circuit inverter transformer using a bar-shaped core is of a simple structure, but has particularly a large leakage inductance, which generates a phase difference in the voltage and the current causing an increase in so-called reactive power, resulting in a substantial decrease in power efficiency.
On the other hand, in a closed-magnetic-circuit inverter transformer, two or more CFLs connected in parallel may all be discharged and turned on. In this case, however, when one CFL starts discharging, and a discharge current flows due to a decrease in the internal impedance of the CFL, thus increasing the load current, then the output voltage of the inverter transformer is reduced despite the small leakage inductance. This may affect discharge conditions of the other CFLs causing variation in the conditions.
Further, since the impedance of the CFLs has negative resistance characteristics, when one CFL starts discharging and turns on, then the impedance of the CFL is rapidly reduced and the current is increased sharply, whereby the inverter transformer may suffer damages, such as winding breakage or the like.
Accordingly, in the closed-magnetic-circuit inverter transformer, since the leakage inductance is small a ballast capacitor Cb is provided between an output terminal T and each of the CFLs 2, as shown in FIG. 25. However, this generates a phase difference between the voltage and the current thereby reducing the so-called reactive power resulting in decreased power efficiency and also invites a cost rise due to increased number of components and due to use of the costly ballast capacitors Cb.
As mentioned above, in the conventional open-magnetic-circuit inverter transformers, the number of inverter transformers increases with the increase in number of CFLs in a 1:1 relationship, thereby increasing the size of the inverter transformer as a whole and pushing up the cost.
In the dosed magnetic circuit structure, one inverter transformer may enable a plurality of CFLs to discharge but it happens that variation occurs in the discharge conditions among the CFLs, or eddy current damages the inverter transformer. The variation in the discharge conditions among the CFLs can be corrected by putting a ballast capacitor in series with each of the CFLs. However, this causes a decrease in power efficiency, an increase in the number of the components and an increase in cost.
The present invention aims to overcome the above problems. The object of the present invention is to provide a compact and less expensive inverter transformer that can simultaneously turn on a plurality of CFLs with a minimum increase in the number of components.
The present invention provides an inverter transformer, which is used in a DC to AC inverter, and adapted to step up an AC voltage inputted to a primary side thereof and to output to a secondary side. The inverter transformer includes an outer core shaped substantially like a rectangular frame, a plurality of inner cores shaped substantially like a bar, a plurality of secondary windings, a primary winding, and a plurality of bobbins shaped substantially like a tube. In the above, the plurality of inner cores are disposed inside the outer core and connected to the outer core so as to have a predetermined leakage inductance. The plurality of secondary windings are provided corresponding to the plurality of inner cores and the primary winding is provided to be common to the plurality of secondary windings. The plurality of bobbins are provided corresponding to the plurality of secondary windings, have the plurality of inner cores inserted therein, respectively, and have the plurality of secondary windings wound thereon, respectively. Furthermore, in the above, the plurality of bobbins each include a primary-side terminal block for the primary winding at one end thereof and a secondary-side terminal block for the secondary winding at the other end thereof are connected together for integration with the secondary windings wound thereon, and have the primary winding wound on the integrated bobbins.
In the above configuration of the present invention, the plurality of bobbins may be integrated such that the primary-side terminal blocks are connected to one another and the secondary-side terminal blocks are connected to one another. The primary-side terminal blocks may each have a projection and a groove for engagement at each connecting portion, and also the secondary-side terminal blocks may each have a projection and a groove for engagement at each connecting portion.
In all of the aforementioned configurations of the present invention, the outer core may be provided with grooves at its side, which engage with parts of the primary-side and secondary-side terminal blocks of the integrated bobbins.
In any one of the aforementioned configurations of the present invention, the primary-side and secondary-side terminal blocks of the integrated bobbins may be provided with projections for engaging with grooves formed on the outer core or with the outer portion of the outer core.
In any one of the aforementioned configurations of the present invention, the inner cores may be each shaped substantially like an L.
In any one of the aforementioned configurations of the present invention, the plurality of bobbins may be shaped identical to one another.